CMOS image sensor and fabricating method thereof

ABSTRACT

A fabricating method of a CMOS image sensor includes the steps of: forming a transfer gate on a semiconductor substrate where a device isolation layer is formed; forming a first n- type ion implantation region for a photodiode beneath a surface of the semiconductor substrate, the first n-type ion implantation region being aligned at one side of the transfer gate and having a first width and a first ion implantation depth; forming a second n-type ion implantation region aligned at one side of the transfer gate, the second n-type ion implantation region enclosing the first n-type ion implantation region and having a second width wider than the first width and a second ion implantation depth deeper than the first ion implantation depth and a second depth; forming a p-type ion implantation region between a surface of the semiconductor substrate and the first n-type ion implantation region, the p-type ion implantation region being aligned at one side of the transfer gate and partially overlapped with the first n-type ion implantation region; forming spacers on both sidewalls of the transfer gate; and forming a floating diffusion region at the other side of the transfer gate.

FIELD OF THE INVENTION

The present invention relates to a CMOS image sensor; and, moreparticularly, to a CMOS image sensor and a fabricating method thereof,in which a second P⁰ ion implantation process is omitted in fabricatinga photodiode, while forming a dual structure of an n-type ionimplantation region for the photodiode, thereby preventing degradationof a device characteristic due to nonuniformity of the second P⁰ ionimplantation and securing charge transport efficiency.

DESCRIPTION OF RELATED ART

An image sensor is an apparatus to convert an optical image into anelectrical signal. Such an image sensor is largely classified into acomplementary metal oxide semiconductor (CMOS) image sensor and a chargecoupled device (CCD).

In the case of the CCD, individual MOS capacitors are disposed. veryclose to one another and charge carriers are stored in the capacitorsand transferred. Meanwhile, in the case of the CMOS image sensor, apixel array is constructed using a technology of fabricating CMOSintegrated circuit and output data are detected in sequence through aswitching operation.

FIG. 1 is a circuit diagram of a unit pixel of a conventional CMOS imagesensor. The unit pixel includes one photodiode 100 and four MOStransistors 101, 103, 104 and 105. The photodiode 100 receives light togenerate photoelectric charges. The transfer transistor 101 transfersthe photoelectric charges accumulated in the photodiode 100 to afloating diffusion region 102. The reset transistor 103 sets a potentialof the floating diffusion region 102 to a desired value and resets thefloating diffusion region 102. The drive transistor 104 serves as asource follower buffer amplifier having a gate receiving a voltage ofthe floating diffusion region 102. The select transistor 105 performs anaddressing function through a switching operation. A load transistor 106is disposed outside of the unit pixel so as to read an output signal.

Among the fabricating processes of the CMOS image sensor, a process offorming the photodiode is the most important in the implementation ofthe optic characteristics. In order to enhance low luminancecharacteristic, a p-type ion implantation region is formed on anuppermost layer of the photodiode by performing a p-type ionimplantation process two times.

FIGS. 2A and 2B are sectional views illustrating a fabricating processof a conventional CMOS image sensor.

Referring to FIGS. 2A and 2B, a device isolation layer 12 is formed on ap-type semiconductor substrate 11 to define an active region and a fieldregion. At this point, the p-type semiconductor substrate 11 can beprovided with a stack structure of a high-concentration p-type substrateand a low-concentration p-type epitaxial layer.

Then, various gate electrodes including a gate electrode of the transfertransistor are patterned. Hereinafter, the gate electrode of thetransfer transistor will be referred to as a transfer gate.

Next, an n-type ion implantation region (or a deep N region) 14 for thephotodiode is formed beneath the semiconductor substrate 11 and isaligned at one side of the transfer gate 13. Thereafter, an ionimplantation process is carried out to form a p-type ion implantationregion (or a Po region) for the photodiode.

That is, the process of forming the n-type ion implantation region (orthe deep N region) 14 for the photodiode is carried out and sequentiallythe p-type ion implantation is carried out using the same mask, suchthat a first p-type ion implantation region 15 is formed on the n- typeion implantation region 14 for the photodiode.

Referring to FIG. 2B, spacers are formed on both sidewalls of the gateelectrode. That is, after a spacer insulating layer is deposited to apredetermined thickness on the semiconductor substrate including thetransfer gate, an entire-surface etching process is carried out to formthe spacers on both sidewalls of the gate electrode.

After the process of forming the spacers, a thin oxide layer 17 remainson the surface of the photodiode. In such a state that the oxide layerremains, a blanket second p-type ion implantation process (second PO) iscarried out without using a mask and thus a second p-type ionimplantation region 18 is formed as shown in FIG. 2B.

Here, the second p-type ion implantation region 18 is formed on then-type ion implantation region 14 and is aligned with the gate spacer 16such that the second p-type ion implantation region 18 is in contactwith the first p-type ion implantation region 15.

In case where the first and second p-type ion implantation regions 18and 15 have the doping profile as shown in FIG. 2B, a fringing fieldhelping the charge transport is generated. Consequently, it isadvantageous to the charge transport efficiency.

However, before the second p-type ion implantation process, thethickness of the remaining oxide layer 17 is different depending on thechips and its variation is very great. Therefore, its uniform adjustmentis difficult.

Because the thickness of the remaining oxide layer serving as the bufferlayer in the ion implantation is not uniform, ion implantation depth ofthe second p-type ion implantation region is also not uniform.Therefore, direct influence on chip characteristics causes the problemsin process stability and yield.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a CMOSimage sensor and a fabricating method thereof, in which a second P⁰ ionimplantation process is omitted in fabricating a photodiode, whileforming a dual structure of an n-type ion implantation region for thephotodiode, thereby preventing degradation of a device characteristicdue to nonuniformity of the second P⁰ ion implantation and securingcharge transport efficiency.

In an aspect of the present invention, there is provided a fabricatingmethod of a CMOS image sensor, including the steps of: forming atransfer gate on a semiconductor substrate where a device isolationlayer is formed; forming a first n-type ion implantation region for aphotodiode beneath a surface of the semiconductor substrate, the firstn-type ion implantation region being aligned at one side of the transfergate and having a first width and a first ion implantation depth;forming a second n-type ion implantation region aligned at one side ofthe transfer gate, the second n-type ion implantation region enclosingthe first n-type ion implantation region and having a second width widerthan the first width and a second ion implantation depth deeper than thefirst ion implantation depth and a second depth; forming a p-type ionimplantation region between a surface of the semiconductor substrate andthe first n-type ion implantation region, the p-type ion implantationregion being aligned at one side of the transfer gate and partiallyoverlapped with the first n-type ion implantation region; formingspacers on both sidewalls of the transfer gate; and forming a floatingdiffusion region at the other side of the transfer gate.

In another aspect of the present invention, there is provided a CMOSimage sensor, including: a transfer gate formed on a semiconductorsubstrate where a device isolation layer; a first n-type ionimplantation region for a photodiode, formed beneath a surface of thesemiconductor substrate, the first n-type ion implantation region beingaligned at one side of the transfer gate and having a first width and afirst ion implantation depth; a second n-type ion implantation regionaligned at one side of the transfer gate, the second n-type ionimplantation region enclosing the first n-type ion implantation regionand having a second width wider than the first width and a second ionimplantation depth deeper than the first ion implantation depth and asecond depth; a p-type ion implantation region formed between a surfaceof the semiconductor substrate and the first n-type ion implantationregion, the p-type ion implantation region being aligned at one side ofthe transfer gate and partially overlapped with the first n-type ionimplantation region; spacers formed on both sidewalls of the transfergate; and a floating diffusion region formed at the other side of thetransfer gate.

In accordance with the present invention, the second p-type ionimplantation process (second Po process) is omitted. Instead, a doublen-type ion implantation region is formed to thereby enhance the chargetransport efficiency and device characteristics such as a dark bad pixelwithout any degradation of device uniformity.

For this, the doping profile and dose of the n-type ion implantationregion formed beneath the spacer of the transfer gate are tuneddifferently from other portions.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the instant invention willbecome apparent from the following description of preferred embodimentstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a circuit diagram of a unit pixel of a conventional CMOS imagesensor;

FIGS. 2A and 2B are sectional views illustrating a method forfabricating a conventional CMOS image sensor; and

FIGS. 3A to 3D are sectional views illustrating a method for fabricatinga CMOS image sensor in accordance with a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described in detail withreference to the accompanying drawings.

FIGS. 3A to 3D are sectional views illustrating a method for fabricatinga CMOS image sensor in accordance with an embodiment of the presentinvention.

Referring to FIG. 3A, a device isolation layer 22 is formed on asemiconductor substrate 21 to define an active region and a fieldregion. A stack structure of a high-concentration semiconductorsubstrate and a low-concentration epitaxial layer can be used for thesemiconductor substrate.

When the stack structure is used, a depth of a depletion layer of aphotodiode formed on the epitaxial layer increases and thus a chargestorage ability of the photodiode increases. Also, thehigh-concentration substrate can prevent a crosstalk occurring betweenadjacent unit pixels.

The device isolation layer 22 can be formed using a local oxidation ofsilicon (LOCOS) technology. In addition, a shallow trench isolation(STI) using a trench can be applied.

Then, a gate insulating layer (not shown) and a gate conductive material23 are sequentially formed on the semiconductor substrate, and a gateelectrode of the transistor is patterned by a patterning process usingan appropriate mask.

In FIG. 3A, only the gate electrode 23 of the transfer transistorcontacting with the photodiode is shown and the remaining transistors(reset transistor or drive transistor) of the unit pixel are not shown.Hereinafter, the gate electrode 23 of the transfer transistor will bereferred to as a transfer gate.

After forming the transfer gate, a first n-type ion implantation region24 for the photodiode is formed at one side of the transfer gate.

It is preferable that a width of the first n-type ion implantationregion 24 for the photodiode is about 0.5 μm and 5) a length thereof isidentical to that of the transfer gate.

Also, the first n-type ion implantation process is carried out under acondition that its dose and ion implantation energy are reduced muchmore than those of the second n-type ion implantation process.

That is, a dose in the first n-type ion implantation process for thephotodiode is smaller than a dose in the second n-type ion implantationprocess by about 0.2×10¹², and the first n-type ion implantation processuses an ion implantation energy of about 60 KeV. Further, theion-implanted depth of the first n-type ion implantation region 24 islower than the conventional n-type ion implantation region.

A mask used to form the first n-type ion implantation region 24 is shownin FIG. 3A. The mask is aligned with the square photodiode, the transfergate and the transfer gate, and its width is about 0.5 μm.

An exposed length of the mask is substantially identical to the lengthof the transfer transistor. In this embodiment, because the mask of FIG.3A is used, the length of the first n-type ion implantation region 24 isidentical to the length of the transfer gate. However, the mask can alsobe longer than the transfer gate.

Referring to FIG. 3B, after the formation of the first n-type ionimplementation region 24, a second n-type ion implantation region 25 isformed.

That is, a second n-type ion implantation region is formed deeplybeneath the substrate and is aligned at one edge ; of the transfer gate23 by using a mask (not shown).

The ion implantation process of forming the second n-type ionimplantation region 25 is a process that is normally used in the priorart. Consequently, the ion implantation regions 24 and 25 can beobtained as shown in FIG. 3B.

Then, a p-type ion implantation process using the same mask is carriedout. That is, a p-type ion implantation region 26 partially overlappedwith the first n-type ion implantation region 24 is formed on the secondn-type ion implantation region 25.

Referring to FIG. 3D, spacers 27 are formed on both sidewalls of thetransfer gate and a floating diffusion region 28 is formed on the otherside of the transfer gate.

Potential distributions of the photodiode region, the transfer gate andthe floating diffusion region when the transfer gate is turned on areshown in FIG. 3D.

In accordance with the present invention, even when the second P⁰ ionimplantation process is omitted, a fringing field advantageous to thecharge transport can be obtained because of the double n-type ionimplantation regions of the photodiode.

Further, the second P⁰ ion implantation process that must be carried outin a state that the non-uniform oxide layer remains is omitted.Therefore, it is possible to prevent inconstant device characteristics,which have been caused by different thickness in the second P⁰ ionimplantation at each chip.

As described above, the present invention can solve the problems of theprocess margin deficiency, the yield variation and the opticalcharacteristic deviation, which are caused by the nonuniformity in thethickness of the oxide layer remaining after etching the spacers.Further, the charge 10 transport efficiency can be secured, therebycontributing to the competitiveness and characteristic stability of theimage sensor.

The present application contains subject matter related to Korean patentapplication No. 2004-32001, filed in the Korean Patent Office on May 6,2004, the entire contents of which being incorporated herein byreference.

While the present invention has been described with respect to theparticular embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. A CMOS image sensor, comprising: a transfer gate formed on asemiconductor substrate where a device isolation layer; a first n-typeion implantation region for a photodiode, formed beneath a surface ofthe semiconductor substrate, the first n-type ion implantation regionbeing aligned at one side of the transfer gate and having a first widthand a first ion implantation depth; a second n-type ion implantationregion aligned at one side of the transfer gate, the second n-type ionimplantation region enclosing the first n-type ion implantation regionand having a second width wider than the first width and a second ionimplantation depth deeper than the first ion implantation depth and asecond depth; a p-type ion implantation region formed between a surfaceof the semiconductor substrate and the first n-type ion implantationregion, the p-type ion implantation region being aligned at one side ofthe transfer gate and partially overlapped with the first n-type ionimplantation region; spacers formed on both sidewalls of the transfergate; and a floating diffusion region formed at the other side of thetransfer gate.
 2. The CMOS image sensor as recited in claim 1, whereinthe first width is 0.5 μm.
 3. The CMOS image sensor as recited in claim1, wherein the first n- type ion implantation region has a lengthidentical to that of the transfer gate.